Portable, or wireless, electronic communication devices have proven to be very useful to society. The widespread use of mobile and portable two-way radios is a good example. Applications for such radios include police, fire, and civil servants, trucking fleets, and industrial intracompany communications. Typically, at least one of the communicating parties is moving or at a location where wired stationary communications is not readily available.
Often such electronic communications devices are used in locations where the atmosphere contains volatile fumes, creating the risk of explosion. Examples of volatile atmospheres include utilities personnel responding to a natural gas pipe rupture, or when a chemical reactor is venting hydrogen gas. Under such conditions the potential exists for the electronics in the area to cause ignition.
To eliminate the risk of ignition, several agencies, such as Factory Mutual in the U.S. and CENELEC in Europe, have established design guidelines allowing designers to make electronic and electrical apparatuses intrinsically safe. That is, the devices and apparatuses have safeguards built in to eliminate the risk of ignition, even under fault conditions. The safety of such devices cannot be compromised by naturally occurring incidents such as dust intrusion or component failure. Accordingly, the device must be designed with expectations of failure in a potentially hazardous atmosphere.
Three parameters that must be considered when designing an intrinsically safe portable electronic device: voltage, current, and power sourced to the device. Voltage is limited to a level based on the maximum capacitance that could be charged, since such a charged capacitance could rapidly discharge were a conductor applied across it's electrodes. The energy of the resulting spark must not be able to cause ignition. The output current to the device is limited to avoid charging the magnetic fields of inductors where an inductance so charged could rapidly break contact and arc; again, the resulting spark must not be energetic enough to cause ignition. Power to the device must be limited so that the surface of any given component cannot heat to an unsafe temperature. Typically this is done by defining the power available as the product of the limited voltage value and the limited current value. The smallest component in the device that could receive this power is then assumed to fail at a resistance that would consume that power, and the maximum surface temperature is then found from that component. It may be possible to reduce the surface temperature exposed to ambient conditions by encapsulating the component, thereby dissipating the heat energy of the component over a larger area.
Temperature classification of the device is dependent on what maximum surface temperature is achievable by any component of the device. The temperature classification of the device is determined at a specific ambient temperature, typically 40.degree. C. If the same device were tested in the same manner at a cooler ambient, the resulting maximum surface temperatures would be likewise be lower. As a result, more power can be sourced to the device at cooler temperature, but power to the device must be more restrictive at temperature above the test ambient of 40.degree. C. The result is that, over temperature, the maximum total power that can be sourced to the device is a linear, negatively sloped function of ambient temperature.
Since a portable electronic device is powered by a battery, the device is not energized until a battery is connected to it. Accordingly, the means by which the voltage and current levels are regulated to a safe level must reside between the device and the battery. If the battery is meant to be detachable while in the presence of a potentially hazardous atmosphere, then such means must reside in the battery itself, and all capacitance, inductance, and surface heat of such means is taken into consideration with the device when determining the safety level. A detachable battery is preferred since the alternative is a battery that can only be removed when in a known safe area. It would be considered an inconvenience to have to travel to a safe area to change batteries. In addition, the semi-permanent type of battery without a safety means could not be carried along with the device since its unregulated output could cause ignition of any volatile gasses encountered.
The amount of capacitance found in many communications devices forces the battery safety means to limit the voltage to a level that may degrade the performance of the device. Given such a limit, it may be impossible to make a given device safe, since device performance is compromised. One possible solution to this problem is to reduce the device capacitance to an acceptable level, and limit the battery voltage to a level slightly in excess of the device's operating voltage threshold. This forces the designer to choose a current limit based either on the device inductance, or based on the thermal characteristics of components in the device that are exposed to the ambient atmosphere. However, since the trend in this field is to make the device small, the components inside are likewise small and can reach very high temperatures when they fail. Accordingly, the battery current limit is usually based on power. This current limit is likely to challenge the designer since it may not allow the device to operate at otherwise maximum settings.
The restrictions of space, cost, and current drain, coupled with the redundancy requirement, dictate that the current limiter is best implemented with discrete components in the form of a simple linear regulator with a single pass device. However, the simplicity of such designs makes them susceptible to temperature effects. Specifically, given an increase in temperature, the bias voltage of a transistor is less than at a cooler temperature. In a simple discrete limiter this has the effect of causing the resulting current limit to be less than what the device demands at higher a temperature. The deviation is attributed to a typical temperature coefficient effect of about -2 mV/.degree.C. of transistor bias voltage.
Referring now to FIG. 1 there is illustrated therein curves of current vs. temperature for the safe power level line 2, maximum current demand of the device line 4, and the limit of a simple discrete limiter line 6. The response of the limiter changes more than the device's demand since the device typically has internal voltage regulators which partially compensate for the change in bias voltage over temperature. As can be seen, at lower temperatures the limiter could allow a level of current such that an unsafe temperature could be reached. By lowering the cool temperature current limit, however, the high temperature performance is degraded since the response of the limiter is less than the demand of the device, as shown in FIG. 2. Consequently designers choose to sacrifice high temperature performance for the sake of safety.
Therefore, there exists a need for a current limiting means that matches the demand of the device while still limiting to a safe current level. The current limiter must more closely follow the maximum current demand change tendency over temperature, thereby providing a safe level of current over the entire temperature range.